1. Field of the Invention
This invention relates to copper alloys having high strength, good formability and relatively high electrical conductivity. More particularly, the yield strength of a tin brass is increased through a controlled addition of iron.
2. Description of Related Art
Throughout this patent application, all percentages are given in weight percent unless otherwise specified.
Commercial tin brasses are copper alloys containing from 0.35% to 4% tin, up to 0.35% phosphorous, from 49% to 96% copper and the balance zinc. The alloys are designated by the Copper Development Association (CDA) as copper alloys C40400 through C49080.
One commercial tin brass is a copper alloy designated as C42500. The alloy has the composition 87%-90% of copper, 1.5%-3.0% of tin, a maximum of 0.05% of iron, a maximum of 0.35% phosphorous and the balance zinc. Among the products formed from this alloy are electrical switch springs, terminals, connectors, fuse clips, pen clips and weather stripping.
The ASM Handbook specifies copper alloy C42500 as having a nominal electrical conductivity of 28% IACS (International Annealed Copper Standard where "pure" copper is assigned a conductivity value of 100% IACS at 20.degree. C.) and a yield strength, dependent on temper, of between 45 ksi and 92 ksi. The alloy is suitable for many electrical connector applications, however the yield strength is lower than desired.
It is known to increase the yield strength of certain copper alloys through controlled additions of iron. For example, commonly owned U.S. patent application Ser. No. 08/591,065 entitled "Iron Modified Phosphor-Bronze" by Caron et al. that was filed on Feb. 9, 1996 and is now U.S. Pat. No. 5,882,442, discloses the addition of 1.65%-4.0% of iron to phosphor bronze. The Caron et al. alloy has an electrical conductivity in excess of 30% IACS and an ultimate tensile strength in excess of 95 ksi.
U.S. Pat. No. 5,882,442 is incorporated by reference in its entirety herein.
Japanese patent application number 57-68061 by Furukawa Metal Industries Company, Ltd. discloses a copper alloy containing 0.5%-3.0%, each, of zinc, tin and iron. It is disclosed that iron increases the strength and heat resistance of the alloy.
Japanese patent application number 61-243141 by Japan Engineering Corp. discloses a copper alloy containing 1%-25% of zinc and 0.1%-5% each of nickel, tin and iron. The alloy further contains 0.001%-1% of boron and 0.01%-5% or either manganese or silicon. The boron and manganese or silicon are disclosed as providing precipitation hardening capability to the alloy.
While the benefit of an iron addition to phosphor-bronze is known, iron causes problems for the alloy. The electrical conductivity of the alloy is degraded and processing of the alloy is impacted by the formation of stringers. Stringers form when the alloy contains more than a critical iron content, which content is dependent on the alloy composition. The stringers originate when properitectic iron particles precipitate from liquid prior to solidification and elongate during mechanical deformation. Stringers are detrimental because they affect the surface appearance of the alloy and can degrade the formability characteristics.
In high copper (in excess of 85% Cu) tin brass, the maximum permissible iron content, as an impurity, is typically 0.05%. This is because iron is known to reduce electrical conductivity and, through the formation of stringers, deteriorate the bend properties.
Copper alloys containing iron and tin within certain compositional ranges exhibit non-dendritic, as-cast, grain structures. For example, U.S. Pat. No. 4,116,686 entitled, "Copper Base Alloys Possessing Improved Processability," by Mravic et al. discloses a copper alloy containing 4.0%-11.0% of tin, 0.01%-0.3% of phosphorous, 1.0%-5.0% of iron and 10 the balance copper. The Mravic et al. alloy may further include small but effective amounts of many specified alloy additions, including zinc. The as-cast alloy is disclosed as possessing a substantially non-dendritic grain structure in the cast condition which contributes to improved processability. The Mravic et al. patent is incorporated by reference in its entirety herein.
Certain non-dendritic alloys have utility as semisolid forming stock. A billet useful as semisolid forming stock has a highly segregated structure consisting of a primary non-dendritic phase surrounded by a segregated phase that melts at a lower temperature than the primary phase. The billet is heated to a temperature effective to melt the lower melting temperature phase, but not the primary phase. If the primary phase is dendritic, the solid primary phase is mechanically locked and no benefit is achieved. If however, the solid primary phase is non-dendritic, then a metal slurry is formed that can be caused to flow under shear stress conditions.
Flowing the slurry into a mold provides a number of advantages over pouring liquid metal of the same composition into the mold. The slurry flows at a lower temperature than required to completely melt an alloy of similar composition. The die is therefore exposed to lower temperatures and die life is increased. The slurry is extruded into a mold with less turbulence than typically results when molten metal is poured causing less air to be entrapped in the casting and therefore, the formed product has less porosity.
Typically, semisolid forming stock is produced by cooling molten metal while the metal is agitated, either mechanically or electromagentically, to fracture dendrites as they form producing a solid phase with substantially spherical degenerate dendrites. U.S. Pat. No. 4,642,146, entitled "Alpha Copper Base Alloy Adapted to be Formed as a Semi-Solid Metal Slurry," by Ashok et al., discloses an alloy useful as semisolid forming stock without stirring or other agitation during casting. The alloy composition is 3%-6% of nickel, 5%-15% of zinc, 2%-4.25% of aluminum, 0.25%-1.2% of silicon, 3%-5% of iron and the balance is copper. A minimum of 3% iron is disclosed for preventing columnar dendrites. The Ashok et al. patent is incorporated by reference in its entirety herein.
It is necessary that the lower melting temperature phase be liquid and the primary, higher melting temperature, phase be solid over a relatively wide temperature range ("semisolid forming processing range"). A wide semisolid forming processing range makes process control easier. For example, an addition of iron to copper alloy C260 (nominal composition of 70% copper and 30% zinc) produced an alloy with only a 5.degree. C. semisolid forming processing range. The alloy exhibited an abrupt transition from initial homogeneous flow (of the slurry) to liquid separation (where molten metal is ejected from the material).
There exists, therefore, a need for an iron modified tin brass alloy that does not suffer from the stated disadvantages of reduced electrical conductivity and stringer formation. There also exists a need for a copper alloy useful as semisolid forming stock that has a broad processing range.